Argon is a component of air that is present at slightly less than 1% mole fraction. Conventional dual pressure processes are employed to separate air at cryogenic temperatures into oxygen and nitrogen. Air is first compressed to approximately 5-6 atm absolute and then subjected to rectification in a high and low pressure distillation column which are thermally linked to one another. The high pressure column operates under superatmospheric pressure corresponding to the pressure of the air feed. The air feed undergoes preliminary separation in the high pressure column into a liquid fraction of crude oxygen and a liquid fraction of substantially pure nitrogen. The two resulting liquids typically form the feed fraction and the rectification reflux for the low pressure distillation operation. Argon is typically recovered through an auxiliary argon sidearm column.
The relative volatilities of nitrogen, argon and oxygen force argon to accumulate in an intermediate stripping section of the low pressure distillation column. An argon enriched gas fraction can be withdrawn from this section to form the feed fraction for the auxiliary or sidearm column which rectifies it. The product vapors exiting the top of the sidearm column form a crude argon stream which is composed primarily of argon, several percent of oxygen and nitrogen in a concentration of typically only 0.005-0.02 mole fraction. An argon condenser supplies the rectification reflux for the sidearm column.
The low pressure column feed is normally the high pressure liquid bottoms. Its composition generally ranges from 34 to 38% oxygen. After partial vaporization in the argon condenser, the kettle liquid is then fed to the low pressure column where the separation is completed, producing a liquid oxygen component collecting in the base of the low pressure column and a gaseous nitrogen component withdrawn from the top of the low pressure column. As an increasing fraction of argon is recovered from the sidearm column the sensitivity of the plant increases to external and internal process flow rate changes and disturbances. Stated otherwise at low argon recovery rates, typically below 10% of the maximum plant recovery rate, argon column sensitivity to process changes is relatively low whereas at high argon recovery rates within 5-10% of the maximum recovery rate for the plant the sensitivity is accentuated and subjects the argon column to a condition where "dumping" may occur. Dumping occurs when the vapor flow up the sidearm column decreases to a point where the gas flow in the sidearm column can no longer support the liquid in the column. A loss of argon recovery is the result of dumping as is the possibility of introducing significant quantities of liquid into the low pressure column which will contaminate the oxygen purity of the low pressure column for a significant period of time. Dumping is therefore a costly economic penalty of the operation at high argon recovery rates. This can always be avoided by purposely recovering sub-optimal levels of argon at recovery rates below 5-10% of the maximum recovery rate which is equivalent to operating at below 75-85% of capacity depending on the plant. However since argon is a highly valued component of air the reduction of argon column product flow is undesirable from an economic standpoint.
High argon recovery levels are normally accompanied by an increase in the nitrogen content of the argon column feed. Accordingly, the maintenance of desirable levels of nitrogen in the feed to the sidearm column is a fundamental problem in the recovery of argon. If there is inadequate control of the nitrogen in the feed to the sidearm column at high argon recovery levels, dumping, as explained earlier, may occur resulting in a loss in argon recovery and in the potential introduction of significant quantities of liquid into the upper low pressure column. Additionally, the argon column will have to be reinventoried. This will also result in the production of off specification material.
The problem of sustaining high argon recoveries has been addressed in the prior art by attempts to control the nitrogen in the argon make. Typically, the nitrogen content in the argon make is of the order of 0.005-0.02 mole fraction and is accordingly measured indirectly by the difference from the concentration measurements of argon and oxygen. The side arm column typically has a large number of rectification stages which results in large liquid holdups within the column and consequently a large apparent deadtime. The large apparent deadtime of the argon column causes the dynamics of the column to act sluggishly or even unstable. The slow dynamics of the column operation limits the effectiveness of any control scheme dependent upon monitoring nitrogen in the argon make. Another method of control is disclosed in U.S. Pat. No. 4,784,677 which is based upon making a direct measurement of the nitrogen content in the argon column feed using a nitrogen analyzer capable of a real time measurement. The patent further teaches a control arrangement based upon using a waste O.sub.2 content measurement from the upper column in conjunction with the real time nitrogen measurement to manipulate the flow of high purity liquid nitrogen reflux to the top of the upper column. The details of the nitrogen analyzer per se is described in U.S. Pat. No. 4,801,209. Since the concentration of nitrogen in the argon column feed is only in parts per million a control methodology dependent upon the accuracy of making real time measurements of variations in nitrogen at this concentration level is not reliable.